1. Field of the Invention
The present invention relates to a process and an apparatus for testing FCC catalysts on a small scale for evaluation of the fluid catalytic cracking processes with particular regard to catalysts, feedstocks, and process parameters.
2. Prior Art
Fluid catalytic cracking is the dominant catalytic process for producing transportation fuels and chemical feedstocks world-wide. Consequently, extensive effort has gone into developing useful small-scale tests pertinent to this process for the purposes of developing improved catalysts, quantifying and correlating the cracking character of various feedstocks based on their respective properties, understanding the implications of different process conditions, and improving commercial process design.
Two broad approaches commonly used in small-scale studies of the fluid catalytic cracking process are continuous processing units and batch processing units. The continuous processing units are basically scaled-down versions (bench or pilot-scale riser reactors) of commercial operating units and are typically very complex systems that are expensive to construct, operate, and maintain. Such a small-scale continuous process test device is described in Chem. Eng. Sci. (1996), 51(11), 3039-3044. In addition, compared to small-scale batch cracking units, such small-scale continuous cracking units require large samples of catalyst and feed. Batch processing units use a single charge of catalyst (typically less than 200 g) and process a small sample mass of feed that is usually injected into the catalyst for a period of time of the order of a minute. The ratio of catalyst mass to feed mass is referred to as the catalyst-to-oil ratio and typically ranges from 3 to 10. Batch processes provide considerable cost and speed advantages over continuous units for laboratory studies, primarily because of their relative simplicity and their smaller scale.
The most commonly used batch process is the so-called microactivity test (MAT). This test is described in ASTM D-3907-86. Said test is the main tool for basic FCC research and catalyst and feedstock evaluation and monitoring. See for instance, Applied Cat., A: General 152 (1997), 7-26, Applied. Cat., A (1997), 164 (1-2), 35-45, Stud. Surf. Sci. Catal. (1997) 111 (catalyst deactivation 1997), 303-310, Catalyst. Cracking, AlChE, Symposium Series (1992) No. 291, Vol. 88, 82-87, and AlChE (1998) Spring National Meeting, New Orleans 3/8-12/8. Despite its widespread use, the possibilities to extrapolate the results obtained from a MAT test to full-scale FCC operations are limited owing to the wide difference between conditions in the MAT test and those in full-scale FCC operation. Below, the conditions in full-scale FCC operation and the conditions in the MAT test are listed.
ASTM-MATFull-scale FCCreactor typefixed bedfluidized bed, riserfeed dispersionnomore than 2%steampreheatnofastinjection time (s)140–1catalyst/feed contact time75 2–10(s).catalyst temperature (° C.)483 650–750pressure (PSIG)atmospheric10–20
As a result of these wide differences in conditions, the MAT test does not give a realistic prediction of the selectivities of catalysts in real FCC units. This has been studied in Applied Cat. 63 (1990) 197-258 and Applied Cat. 43 (1988) 213-237. For this reason other tests have been developed, such as the microsimulation test (MST) from Akzo Nobel as described in J. Am. Chem. Soc., Div. Pet. Chem. (1988) 33(4), 656-62 and in ACS. Symp. Series, No. 411, 135-147 (1989). In this test a catalyst/feed contact time of 15 seconds is realized, which is more in line with real FCC processing than the catalyst/feed contact time of 75 seconds in the ASTM-MAT test. In Hydrocarbon Processing, Sept. 1989, 63-4, a microactivity test is described with a contact time of 18 seconds and a cracking temperature of 510° C.
In Chem. Eng. Sci. 64 (1999) a bench-scale FCC test device is described with very short contact times (50-500 ms), but this device is a downflow device using catalyst temperatures of about 400-600° C. and thus not a proper simulation device for real FCC units, which contain upflow riser reactors.
In U.S. Pat. No. 6,069,012 a laboratory-scale fluid catalytic cracking apparatus is described. Said apparatus includes a reactor having a removable feed injector to vary the catalyst/feed contact time. The feed injector is inserted downwards into the catalyst bed. Relatively large amounts of catalyst (9 g in the examples) are used in this apparatus. The injection time of this apparatus is not mentioned. Although it is indicated that the catalyst bed is fluidized, it is a so-called slow, fixed fluidized bed which does not resemble the fast fluidized beds in a full-scale FCC unit.
In the design illustrated in Johnson, P. H. et al, Journal of Industrial and Engineering Chemistry, 1953, 45, pages 849-562 there is shown an apparatus for a laboratory test method by which stripping and oil go via the same line, via the oil preheater, towards the reactor. The oil is not directly injected into the reactor but is stripped by nitrogen towards the oil preheater and then at least partly evaporates before it enters the reactor.
In the paper O'Connor et al., Accessible FCC Catalysts for Short Contact Time Cracking; Prepa. Am. Chem., Soc. Div. Pet. Chem (1966) Pgs. 359-360, a Micro Simulation Test is described, with short injection and contact times, but a very large amount of inert gas (volume ratio of inert gas to hydrocarbon greater than 50) was employed to obtain good feed-catalyst mixing and to control the reactor riser temperature.
Although the microsimulation tests give a more accurate prediction of catalyst selectivity in general, with the necessity to process heavier feedstocks, to obtain higher gasoline motor octanes, and to fulfil the environmental specifications on NOx, SOx as well as sulfur levels in gasoline, there is an ongoing need for reproducible tests which also accurately simulate resid FCC operations, operations at higher riser temperatures, operations with higher catalyst to oil ratios, etc.